Laminate containing coated polyester film

ABSTRACT

A bonding material is described that is well suited to bonding polymer films, such as polyester films, to other substrates. In one embodiment, for instance, the bonding material can be used to bond a polyester film to a layer containing an ethylene-vinyl acetate copolymer to form a backing material for a photovoltaic device. The bonding material generally comprises a polymer component combined with a cross-linking agent. The polymer component may comprise an alkylene carboxylic acid copolymer, such as an ethylene acrylic acid copolymer or an ethylene methacrylic acid copolymer. In an alternative embodiment, the polymer component may comprise a phenoxy resin. The cross-linking agent, on the other hand may comprise an oxazoline polymer.

RELATED APPLICATIONS

The present application is based and claims priority to U.S. PatentApplication Ser. No. 61/547,286 filed on Oct. 14, 2011, which isincorporated herein by reference.

BACKGROUND

Photovoltaic devices convert light energy, such as sunlight, intoelectricity. Photovoltaic devices typically include one or more solarcells that contain a semiconductor material. The semiconductor materialmay comprise, for instance, silicon or any other suitable material. Whenlight energy strikes the solar cell, the semiconductor material canproduce an electric current if connected to an electric circuit. Anumber of solar cells electrically connected to each other and mountedin a support structure are typically referred to as a photovoltaicmodule. A photovoltaic array, on the other hand, can comprise aplurality of modules.

The solar cells are typically placed behind a transparent material, suchas a glass sheet. These electronic components are typically encapsulatedbetween sheets of transparent polymer, most frequently by a chemicallycross-linked ethylene-vinyl acetate (EVA) copolymer. On the oppositeside, the solar cell typically includes a backing material that isintended to electrically isolate the electronic components as well asprotect them from environmental degradation. The backing material, forinstance, should exhibit moisture resistance and/or heat resistance. Thebacking material should also be capable of providing some structure andshould comprise a material having electrical insulation properties.

In the past, the backing material has comprised various different typesof laminates. In one embodiment, for instance, the backing material hascomprised an ethylene-vinyl acetate copolymer layer laminated to apolyester film. The ethylene-vinyl acetate copolymer layer exhibitsexcellent adhesion to the cross-linked EVA encapsulant layer. Thepolyester film, on the other hand, provides environmental protection andis a very good electrical insulator. Problems have been experienced,however, in achieving a strong and hydrolytically stable adhesive bondbetween the polyester film and ethylene-vinyl acetate copolymer layer.

In the past, in order to improve the adhesion between the two layers,various different adhesives have been proposed. For example, variousprior art constructions are disclosed in U.S. Patent ApplicationPublication No. 2008/0050583 and Japanese Patent Application No.2006-332091, which are both incorporated herein by reference. U.S.Patent Application Publication No. 201010215902, which is alsoincorporated herein by reference, discloses a white adhesive primedpolyester film for improved adhesion to the cross-linked ethylene-vinylacetate encapsulant layer of a solar cell module. The '902 applicationhas provided great advancements in the art. Further improvements,however, are still needed.

SUMMARY

The present disclosure is generally directed to a coating that can beapplied to a polyester film for improving adhesion between the polyesterfilm and a second polymeric layer. The invention is particularlyeffective when utilized to bond a polyethylene terephthalate sheet to across-linked ethylene-vinyl acetate copolymer. Of particular advantage,the adhesive coating is particularly resistant to degradation caused byweathering, and is capable of retaining a relatively large amount of itsinitial bond strength between the two layers even after exposure formany hours to an environment at a relatively high temperature and atrelatively high humidity levels. The coated polyester film isparticularly well suited for use in constructing backing materials forsolar cells, solar modules and solar arrays. It should be understood,however, that the coated polyester film also has numerous other uses.

Of particular advantage, the coating composition of the presentdisclosure has also demonstrated excellent adhesion to polyester filmsthat contain various fillers. For example, in one embodiment, thepolyester film may contain a significant amount of while pigmentparticles, such as barium sulfate particles. The white pigment particlesare added to the film, in one embodiment, to provide the film with awhite color, such that the film has a Berger whiteness of greater than70, such as greater than 75, such as greater than 80. The white pigmentparticles are included in the film so that the film has desiredreflectance properties, especially when the film is used in conjunctionwith a photovoltaic device. The white pigment particles, however, mayhave a tendency to interfere with the ability of a coating to bond thefilm to an opposing film or surface. The coating composition of thepresent disclosure, however, has been found to be an effective bondingagent even when the polyester film contains the white pigment particles.

In general, the coating applied to the polyester film comprises apolymer component combined with a cross-linking agent. The polymercomponent may comprise, for instance, an alkylene carboxylic copolymersuch as an ethylene acrylic acid copolymer, an ethylene methacrylic acidcopolymer, or a butylene acrylic acid copolymer. In an alternativeembodiment, the polymer component may comprise a phenoxy resin. Thecross-linking agent, on the other hand, may comprise an oxazolinepolymer, a carbodiimide polymer, an epoxy, an isocyanate, or a melamine.In one embodiment, for instance, the cross-linking agent comprises anoxazoline polymer.

In one embodiment, the polyester film is at least uniaxially stretched.For example, in one embodiment, the film can be biaxially stretched. Inorder to form the coating on the first side of the film, a coatingcomposition is dispersed in a liquid carrier and can be applied to thefilm prior to complete stretching of the film. For instance, in oneembodiment, the coating dispersion can be applied prior to stretchingthe film in the cross-direction. During stretching, the coatingcomposition is heated and consolidated to form a coating on the film.

In addition to a coated film, the present disclosure is also directed toa laminate comprising a polyester film bonded to a second polymer film.The second polymer film, in one embodiment, contains a cross-linkableethylene-vinyl acetate copolymer. In other embodiments, however, thesecond polymer film may contain various other polymers. For example, thesecond polymer film may comprise a polyolefin polymer, such as a lowdensity polyethylene, polypropylene, or copolymers thereof. In stillother embodiments, the second polymer film may comprise an ionomer ormay comprise a thermoplastic elastomer. The laminate, for instance, maybe used as a backing material for solar cells. In accordance with thepresent disclosure, the polyester film is attached to the second polymerfilm by applying the coating between the two layers as described above.

The coating or bonding layer is capable of forming strong bonds betweenthe polyester film and the second polymer film. For instance, theinitial bond strength between the polyester film and the second polymerfilm can be at least about 60 N/25 mm.

In addition to having excellent initial bond strength characteristics,the bonding layer is also capable of retaining its bond strength betweenthe layers even after exposure to relatively high temperatures and torelatively high humidity levels for extended periods of time. Forexample, in one embodiment, the bond strength between the polyester filmand the second polymer film is at least about 25% of the initial bondstrength, such as at least about 40% of the initial bond strength afterexposure to an environment at 100% relative humidity at 121° C. for 24hours.

The above described laminate can be used as a backing layer forphotovoltaic devices. In one embodiment, for instance, the photovoltaicdevice includes a layer comprised a semiconductor material that iscapable of converting light energy into an electric current. Thesemiconductor material can be covered by a transparent panel, such as aglass panel that allows light energy to reach the semiconductor materialwhile providing protection. On the opposite side of the semiconductormaterial, the photovoltaic device can include a backing layer comprisingan adhesively primed PET sheet or laminate as described above.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a perspective view of one embodiment of a photovoltaic devicethat may be made in accordance with the present disclosure;

FIG. 2 is a plan view of the back side of the photovoltaic device shownin FIG. 1;

FIG. 3 is a cross-sectional view of one embodiment of a photovoltaicdevice including a composite backing layer made in accordance with thepresent disclosure;

FIG. 4 is a cross-sectional view of one embodiment of a coated polymerfilm made in accordance with the present disclosure; and

FIGS. 5A and 5B are a plan view and a side view respectively of a samplepreparation for conducting the peel test as described hereinafter.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

In general, the present disclosure is directed to a coated polymer filmand particularly to a coated polyester film. The coating contained onthe film is for attaching the film to other substrates or layers. Forexample, the coating is particularly well suited for attaching apolyester film to a layer containing a cross-linked ethylene-vinylacetate copolymer. Cross-linkable ethylene-vinyl acetate compositionsare known to those skilled in the art. Exemplary examples include F806EVA offered by Hangzhou First PV Material Company and Photocap 14520P/UFoffered by Specialized Technology Resources, Inc. Although the coatedfilm may be used in numerous applications, in one embodiment, the filmcan be laminated to a layer containing a cross-linked ethylene-vinylacetate copolymer and used as a composite backing material for aphotovoltaic device.

Referring to FIG. 1, for exemplary purposes only, a photovoltaic device10 is illustrated. The photovoltaic device 10 can include one or moresolar cells that are well suited to converting light energy, such assunlight, into electricity.

In the embodiment illustrated in FIG. 1, the photovoltaic device 10includes a semiconductor material 12 contained within a frame 14. Thesemiconductor material 12 is contained behind a transparent protectivelayer, such as a glass layer. An anti-reflective coating can bepositioned in between the glass layer and the semiconductor material.The anti-reflective coating may be present in order to reduce the amountof light that is reflected and not used by the photovoltaic device. Thetransparent cover plate protects the device from the elements.

As shown, the semiconductor material 12 is divided into individualelements which may be considered individual solar cells. Thesemiconductor material 12 can be made from any suitable material capableof converting light energy into electricity. For instance, suitablematerials that may be used to form the semiconductor material 12 maycomprise crystalline or amorphous silicon, alloys ofcopper-indium-gallium-selenide (CIGS), cadmium telluride, galliumarsenide, or blend of organic semi-conducting materials.

In one embodiment, the semiconductor material 12 may comprise twodifferent layers: (1) an N-type semiconductor material layer, and (2) aP-type semiconductor material layer. For example, in one embodiment,both layers can be made from silicon. The two layers form a P-N junctionphotodiode. When exposed to sunlight, electrons may flow from the P-typelayer to the N-type layer. When connected to an external current path,an electric current is then established, which can be conducted awayfrom the photovoltaic device and used as desired. In this regard, thephotovoltaic device can include a contact grid that establisheselectrical connections.

Photovoltaic devices such as the one illustrated in FIG. 1 and variousother solar panels are typically placed in outside environments atlocations where there is ample amount of sunlight. Such environmentstypically have warm to hot temperatures. The environments can also berelatively humid. In order to protect the photovoltaic device from theelements, in addition to a top transparent plate, the devices typicallyinclude a composite backing material. In the past, backing materialshave included a layer containing an ethylene-vinyl acetate copolymerlaminated to a polymer film, such as a polyester film. The backingmaterial electrically isolates the active components of the solar cellfrom the conducting wires and ground, and protects the internalcomponents from environmental degradation and mechanical abuse. Thebacksheet layer can also act to reflect incident light back onto theactive components, further increasing the efficiency of the device.

Referring to FIG. 2, for instance, the back side of the photovoltaicdevice 10 is shown. As illustrated, the photovoltaic device includes acomposite backing material 16 contained within the frame 14. In thepast, various problems have been experienced in securely bonding polymerfilms, such as polyester films, to other polymeric films. In thisregard, the present disclosure is directed to a bonding layer that canbe placed between the two materials for greatly improving adhesion,especially when the backing material is subjected to higher temperaturesat humid conditions for extended periods of time.

In addition to displaying excellent adhesion properties at hightemperatures and under humid conditions, the bonding layer of thepresent disclosure is also is capable of bonding together a polyesterfilm that contains a relatively great amount of inorganic pigmentparticles to an opposing polymer film. For example, in the past, loadingthe polyester film with desired amounts of pigment particles and otherfiller particles resulted in a surface morphology with a diminishedability to form bonds with an adhesive coating or other polymer layer.White pigment particles are typically added to the polyester film sothat the film will reflect light when used with a photovoltaic device.Of particular advantage, the bonding layer of the present disclosure isparticularly well suited for bonding polyester films to other polymerfilm layers even when the polyester film contains pigment particles,such as barium sulfate particles.

In one embodiment, the bonding material of the present disclosure can beapplied to the polymer film as a coating prior to laminating the film toa polymeric layer. In general, the bonding material can be used to bondnumerous different polymeric films together. For example, in oneembodiment, the bonding material can be used to bond together apolyester film to a layer containing a cross-linked ethylene-vinylacetate copolymer. In other embodiments, the polymeric layer maycomprise any suitable thermoplastic polymer, such as a polyolefin. Forinstance, the polymeric layer may comprise a polyethylene, such aslinear low density polyethylene, a polypropylene, mixtures thereof, andcopolymers thereof. In other embodiments, the polymeric layer maycontain one or more lonomers or elastomers.

The bonding material of the present disclosure exhibits excellentinitial bond strength to both PET and the second polymer layer, as wellas excellent retained bond strength after exposure to highly acceleratedstress test (HAST) conditions. It is believed that good initial bondstrength is achieved due to good mechanical and chemical compatibilitybetween the bonding material and polymer layer as well as many strongcovalent bonds with both the polymer layer and the surface of the PET.HAST testing is conducted to simulate over a short time frame the longterm effects of environmental aging. It is believed that the excellentretained bond strength after HAST testing results from the strong,hydrolytically stable bonds produced when the coating reacts with thesurface of the PET and the polymer layer. Further, it is believed thatthe hydrophobic nature of the components utilized in the bonding layerminimize the solubility and concentration of water molecules at bondinginterfaces during HAST aging, further enhancing the hydrolytic stabilityof the bonding interfaces.

The bonding material of the present disclosure exhibits excellentinitial bond strength. For example, the initial bond strength or peelstrength between the polymer film layer and the layer containing thecross-linked ethylene-vinyl acetate copolymer can be greater than about50 N/25 mm, such as greater than about 60 N/25 mm, such as greater thanabout 70 N/25 mm. In one embodiment, for instance, the initial bondstrength can be from about 35 N/25 mm to about 80 N/25 mm.

In order to determine the peel strength of a bonding material inaccordance with the present disclosure, a sample as shown in FIGS. 5Aand 5B is first prepared. As illustrated, the sample includes across-linkable ethylene-vinyl acetate layer 54 positioned in between twopolyester film layers 50 and 52. A release sheet 56 is placed in betweenthe polyester film layer 50 and the ethylene-vinyl acetate layer 54. Therelease sheet can comprise, for instance, a fabric coated with afluorocarbon such as TEFLON polymer.

Once the layers are brought together as shown in FIGS. 5A and 5B, thesample is placed in vacuum press for lamination. The laminate issubjected to a vacuum for 1.5 minutes, and then sample is held undervacuum as 1 bar of atmospheric pressure is applied for 19.5 minutes.During this cycle the laminate is heated from room temperature to 150°C., and held at 150° C. for approximate 18 minutes. During this heatingcycle, the cross-linking component of the EVA is activated, and the EVAlayer is cross-linked. At the end of the cycle, the press is opened andthe laminate is removed and cooled.

Once the laminate sample is prepared as shown in FIG. 5A, it is cut into25 mm wide test strips. By including the release sheet 56 in the sample,an adhesion-free edge of the polyester film 50 is obtained in each teststrip.

Each test strip is then placed in an Instron tensile testing machine. Inparticular, the free edge of the polyester film 50 where the releasesheet 56 is located is placed in one jaw and the opposing portion of thesample comprised of the ethylene-vinyl acetate layer 54 and thepolyester film 52 is placed in the opposite jaw. An aluminum backingplate is fixed in the upper grip adjacent to the upper portion of thePET laminate. This plate hangs down behind the peel specimen and forcesthe tab comprising the adhesively bonded material to bend down parallelto the clamped tabs. The Instron machine is set at a rate of 100 mm perminute, and a 180° peel test is performed. Average peel force isrecorded when steady state conditions are achieved. The machineindicates a peel strength in Newtons of force per 25 mm of laminatewidth. At least 6, but typically 8-10 peel tests are conducted for eachlaminate to determine an average peel force.

In addition to displaying good initial bond strengths, the bondingmaterial of the present disclosure is also capable of retaining asignificant portion of the initial bond strength even when subjected toHAST aging. For instance, the bonding material is capable of retainingat least 25% of its initial bond strength even when exposed to anenvironment at 121° C. and 100% RH for 24 hr. For instance, the bondingmaterial may retain greater than 30%, such as greater than 40%, such aseven greater than 50% of its initial bond strength when subjected to theabove conditions.

Referring to FIG. 3, one embodiment of a backing material 16 made inaccordance with the present disclosure is illustrated. As shown, thebacking material 16 is positioned adjacent to the cross-linked EVA whichencapsulates the semiconductor material 12 contained within aphotovoltaic device.

As illustrated, the backing material 16 a polymer film 18, which maycomprise a polyester film. In order to attach the polymer film 18 to thelayer 20, a bonding layer 22 is positioned in between the layer 20 andthe polymer film 18. As shown in FIG. 4, in one embodiment, the bondinglayer 22 may comprise a coating which is applied to the polymer film 18prior to laminating the film to the layer containing the ethylene-vinylacetate copolymer.

In accordance with the present disclosure, the bonding layer 22comprises a polymer component combined with a cross-linking agent. Inone embodiment, the polymer component comprises an alkylene carboxylicacid copolymer. For instance, the alkylene carboxylic acid copolymer maybe ethylene based, butylene based, or propylene based. Particularexamples of alkylene carboxylic acid copolymers include ethylene-acrylicacid copolymers, ethylene-methacrylic acid copolymers, andbutylene-acrylic acid copolymers.

In an alternative embodiment, the polymer component may comprise aphenoxy resin.

As described above, the polymer component is combined with across-linking agent. The cross-linking agent comprises at least onewater-miscible or water-dispersible component which bears reactivegroups which can enter into cross-linking reactions, for examplepolymers with oxazoline groups, carbodiimide groups, epoxy groups,isocyanate groups or melamine. As used herein, the above polymers arereferred to as oxazoline polymers, carbodiimide polymers, epoxies,isocyanates, or melamines. Among these, especially polymers withoxazoline or carbodiimide groups are preferred.

Polymers containing oxazoline groups are macromolecular compounds whichform through addition polymerization of a) oxazoline derivativesaccording to the structural formulae (I) to (III) and b) at least onefurther comonomer.

In the structural formulae (I) to (Ill) depicted above, the R1, R2, R3and R4 radicals may each independently represent hydrogen atoms, halogenatoms, alkyl groups, aralkyl groups, phenyl groups or substituted phenylgroups. R5 is a noncyclic radical which contains a polymerizable doublebond.

Examples of halogen atoms are fluorine, chlorine, bromine and iodine,preference being given to chlorine and bromine. Examples of alkyl groupsare methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl andhexyl groups. Aralkyl groups are understood to mean those radicals whichcontain alkyl groups with a chain length of 1 to 5 carbon atoms, forexample benzyl, phenethyl, benzhydryl and naphthylmethyl groups.Substituted phenyl groups may, for example, be chlorophenyl,bromophenyl, methoxyphenyl, ethoxyphenyl, methylaminophenyl,ethylaminophenyl, dimethylaminophenyl, methylethylaminophenyl anddiethylaminophenyl. Examples of noncyclic radicals with polymerizabledouble bonds are vinyl and isopropenyl groups.

Examples of oxazoline derivatives a) include 2-vinyl-2-oxazoline,2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline,2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline and2-isopropenyl-5-ethyl-2-oxazoline. To prepare polymers bearing oxazolinegroups, it is possible to use the oxazoline derivatives a) eitherindividually or in a combination of two or more of the compoundsmentioned. Among the oxazoline derivatives mentioned,2-isopropenyl-2-oxazoline is particularly preferred.

The comonomers b) used may in principle be all compounds which arecopolymerizable with oxazoline derivatives a). Examples of comonomers b)are methacrylic esters such as methyl methacrylate, butyl methacrylateand 2-ethyihexyl methacrylate, unsaturated carboxylic acids such asmethacrylic acid, itaconic acid and malonic acid, unsaturated nitrilessuch as methacrylonitrile, unsaturated amides such as methacrylamide andN-methylolmethacrylamide, vinyl esters such as vinyl acetate and vinylpropionate, vinyl ethers such as methyl vinyl ether, ethyl vinyl ether,alpha-olefins such as ethene, propene, halogenatedalpha,beta-unsaturated compounds such as vinyl chloride, vinylidenechloride and vinyl fluoride, and also alpha,beta-unsaturated aromaticcompounds such as styrene and alpha-methyistyrene. The comonomers b)mentioned may be used either individually or in a combination of 2 ormore compounds.

The polymer containing oxazoline groups can be prepared, for example, byadding an oxazoline derivative a), at least one comonomer b) and afree-radical initiator, for example benzoyl peroxide orazoisobutyronitrile, to a suitable water-miscible organic solvent andheating the resulting solution. After the polymerization has ended,water can be added and the organic solvent can be removed completely orpartially by distillation, which leaves an aqueous polymer dispersioncontaining oxazoline groups, which is directly suitable for preparationof the inventive coating solution.

Alternatively, it is also possible to polymerize oxazoline derivative(s)a) and comonomer(s) b) anionically, for example with n-butyllithium.

The content of oxazoline groups in the dried polymer is typically 0.5 to10 mmol/g, preferably 1.5 to 8 mmol/g. The glass transition temperatureof the dried polymer is in the range between 0 and 100° C., preferably20 to 95° C.

Suitable aqueous polymer dispersions containing oxazoline groups arecommercially available under the name “EPOCROS®” from Nippon Shokubai(Japan). In this context, water-soluble, solvent-free products of the“EPOCROS®WS” series from the abovementioned manufacturer areparticularly suitable for the inventive coating solution.

Polymers containing carbodiimide groups are macromolecular compoundswhich bear at least two carbodlimide groups per molecule and which canbe prepared by polycondensation of diisocyanates in the presence ofcatalysts. Corresponding processes are prior art and are described,inter alia, in EP-A-0 878 496 (whose United States equivalent is U.S.Pat. No. 6,124,398). Suitable starting materials for preparing polymerscontaining carbodlimide groups are aromatic, aliphatic and alicyclicdiisocyanates, for example toluene diisocyanate, xylene diisocyanate,diphenylmethane diisocyanate, hexamethylene diisocyanate, cyclohexanediisocyanate, isophorone diisocyanate and dicyclohexyl diisocyanate.Polymers containing carbodiimide groups may also contain surfactants,polyalkylene oxides or hydrophilic monomers, for example quaternaryammonium salts, dialkylamino alcohols and hydroxyalkylsulfonic acid, inorder to improve the solubility or dispersibility.

Polymers containing epoxy groups are, for example,bisphenol-epichlorohydrin-based polymers, cycloaliphatic polymericepoxides, epoxy compounds based on Novolac, epoxy-olefin polymers, epoxycompounds based on polyol-glycidyl compounds and epoxysilane polymers.Especially suitable are polyethylene glycol diglycidyl ether,polypropylene glycol diglycidyl ether, neopentyl glycol diglycidylether, 1,6-hexaneglycol diglycidyl ether, glyerol polyglycidyl ether,trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether andbisphenol A diglycidyl ether.

Polymers containing isocyanate groups are polyisocyanates, for example2,4-toluene diisocyanate, 2,6-toluene diisocyanate, m-phenylenediisocyanate, p-phenylene diisocyanate, 4,4’-diphenylmethanediisocyanate, 2,4′-diphenylmethane diisocyanate, 2,2′-diphenylmethanediisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate,3,3′-dimethoxy-4,4′-biphenylene diisocyanate,3,3′-dichloro-4,4′-biphenylene diisocyanate, 1,5-naphthalenediisocyanate, 1,5-tetrahydronaphthalene diisocyanate, tetramethylenediisocyanate, 1,6-hexamethylene diisocyanate, dodecamethylenediisocyanate, trimethylhexamethylene diisocyanate, 1,3-cyclohexylenediisocyanate, 1,4-cyclohexylene diisocyanate, xylene diisocyanate,tetramethylxylylene diisocyanate, hydrogenated xylene diisocyanate,lysine diisocyanate, isophorone diisocyanate, 4,4′-dicyclohexyldiisocyanate, 3,3-dimethyl-4,4′-dicyclohexylmethane diisocyanate,tetramethylxylene diisocyanate, and polymers with isocyanate end groupsfrom the reaction of the abovementioned compounds with a trifunctionalpolyisocyanate of the isocyanurate or biuret type, or a dihydric orhigher polyhydric polyol.

Melamine is understood to mean compounds which can be prepared by thereaction of methylolmelamine derivatives, obtainable by condensation ofmelamine and formaldehyde with lower alcohols, for example methanol,ethanol and isopropanol (or mixtures of these alcohols). Examples ofmethylolmelamine derivatives are monomethylolmelamine,dimethylolmelamine, trimethylolmelamine, tetramethylolmelamine,pentamethylolmelamine and hexamethylolmelamine.

In addition to the polymer component and the cross-linking agent, thecoating composition may contain various other ingredients. In oneembodiment, for instance, the coating may contain one or moreantiblocking agents. For example, the antiblocking agent may compriseinorganic and/or organic particles. Particular examples of antiblockingagents include silicone dioxide, calcium carbonate, and aluminum oxide.

The relative amounts of the components in the bonding layer or coatingcan vary depending upon various factors. In one embodiment, forinstance, the polymer component may be contained in the coating orbonding layer in an amount from about 40% to about 99% by weight, suchas from about 60% to about 99% by weight. The coupling agent, on theother hand, may be present in the coating in an amount from about 0.01%to about 40% by weight, such as from about 0.1% to about 30% by weight.

The dried coating on the polyester film can generally have a thicknessfrom about 5 nm to about 500 nm. For instance, the dried coating canhave a thickness from about 10 nm to about 100 nm, such as from about 10nm to about 60 nm. The polyester film, on the other hand, can typicallyhave a thickness of from about 5 mils to about 15 mils.

The bonding layer may be formed on the polymer film using any suitabletechnique or method. In one embodiment, the components of the bondinglayer are contained in an aqueous composition and applied to the polymerfilm while the polymer film is being formed. The coating composition,for instance, can have about 5% to about 30% solids, such as from about12% to about 25% solids.

in the embodiment illustrated in FIG. 4, only one side of the polymerfilm 18 is coated with the bonding layer 22. It should be understood,however, that in other embodiments both sides of the polymer film may becoated. In this manner, the polymer film can be laminated to the same ordifferent substrates on either side. When forming photovoltaic devices,for instance, it may be useful to coat both sides of the polymer film inorder to bond the film to a layer containing an ethylene-vinyl acetatecopolymer on one side and to various electrical devices, such as ajunction box, on the opposing side. The junction box, for instance, maybe used in order to direct the electric current being created by thephotovoltaic device to a downstream end use.

The polymer film 18 as shown in FIG. 4 can generally comprise anysuitable polymer. For instance, polyester films are particularly wellsuited for use in the present disclosure. The polyester used toconstruct the film may comprise polyethylene terephthalate, polyethylenenaphthalate or polybutylene terephthalate. The polymer film may alsocomprise copolyesters, such as polyethylene terephthalate isophthalate.Generally, any polyester film based on a polymer resulting frompolycondensation of a glycol or diol with a dicarboxylic acid (or itsester equivalent) such as terephthalic acid, isothalic acid, sebacicacid, malonic acid, adipic acid, azelaic acid, glutaric acid, subericacid, succinic acid, or mixtures thereof. Suitable glycols includeethylene glycol, diethylene glycol, polyethylene glycol, and polyolssuch as butanediol and the like. Mixtures of two or more of theforegoing are also suitable.

Any of the above based polymer films can contain conventional additivessuch as antioxidants, delusterants, pigments, fillers such as silica,calcium carbonate, kaolin, titanium dioxide, antistatic agents and thelike or mixtures thereof. In one embodiment, for instance, a filler maybe present in the polymer film sufficient to colorize the film andincrease the opacity of the film. In one embodiment, for instance, thefilm can include a filler to make the film have a white appearance. Onefiller that may be used, for instance, is barium sulfate. Barium sulfatemay be present in the film in an amount from about 5% to about 30% byweight, such as from about 15% to about 25% by weight.

For a further increase in the whiteness, suitable optical brightenerscan optionally be added to the pigmented film (in a multilayer structurepreferably to the pigmented layers). Suitable optical brighteners are,for example, HOSTALUX® KS (from Clariant, Germany) or EASTOBRIGHT® OB-1(from Eastman, USA),

The film may comprise further particles as antiblocking agents in one ormore layers. Typical antiblocking agents are inorganic and/or organicparticles, for example silicon dioxide (precipitated or fumed), calciumcarbonate, magnesium carbonate, barium carbonate, calcium sulfate,lithium phosphate, calcium phosphate, magnesium phosphate, kaolin(hydrated or calcined), aluminum oxide, aluminum silicates, lithiumfluoride, calcium salts, barium salts, zinc salts or manganese salts ofthe dicarboxylic acids used, or cross-linked polymer particles, forexample polystyrene or polymethyl methacrylate particles.

In addition, it is also possible to select mixtures of two or moreparticle systems or mixtures of particle systems with the same chemicalcomposition but different particle size as antiblocking agent.

When particles are present as antiblocking agents in a layer of thefilm, the total concentration of these particles is less than 20% byweight, based on the total weight of the modified layer, preferably lessthan 15% by weight and more preferably less than 5% by weight. Theparticles have a mean size of 0.01 to 15 μm, preferably 0.03 to 10 μmand more preferably 0.05 to 5 μm.

The film may comprise further additives such as UV stabilizers, flameretardants, hydrolysis stabilizers and antioxidants.

UV stabilizers, i.e. UV absorbers as light stabilizers, are chemicalcompounds which can intervene in the physical and chemical processes oflight-induced polymer degradation. Suitable UV stabilizers are, forexample, 2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickelcompounds, salicylic esters, cinnamic ester derivatives, resorcinolmonobenzoates, oxanilides, hydroxybenzoic esters, benzoxazinones,sterically hindered amines and triazines, preference being given to the2-hydroxybenzotriazoles, the benzoxazinones and the triazines. In a veryparticularly preferred embodiment, the inventive film comprises, as a UVstabilizer, 0.1 to 5.0% by weight of2-(4,6-diphenyl[1,3,5]triazin-2-yl)-5-hexyloxyphenol of the formula

or 0.1 to 5.0% by weight of2,2′-methylenebis[6-benzo-triazol-2-yl]-4-(1,1,2,2-tetramethylpropyl)phenolof the formula

or 0.1 to 5.0% by weight of2,2′-(1,4-phenylene)bis([3,1]benzoxazin-4-one) of the formula

In a further embodiment, it is also possible to use mixtures of these UVstabilizers or mixtures of at least one of these UV stabilizers withother UV stabilizers, where the total concentration of light stabilizersis preferably between 0.1 and 5.0% by weight, more preferably in therange from 0.5 to 3.0% by weight, based on the weight of the film.

The films may be produced by any well known technique in the art. Forexample, polyester is typically melted and extruded as an amorphoussheet onto a polished revolving casting drum to form a cast sheet of thepolymer. The sheet is quickly cooled and then stretched or oriented inone or more directions to impart strength and toughness to the film. Forinstance, the sheet can be uniaxially stretched or biaxially stretched.

During extrusion, the temperature of the film is generally below about300° C. For instance, the temperature during extrusion can be from about275° C. to about 295° C.

Stretching of the film can generally occur as the film is beingproduced, although stretching can also be conducted offline. Biaxialstretching, for instance, is generally carried out in succession, butcan take place simultaneously. When done in succession, stretchingtypically first takes place longitudinally (in the machine direction)and then transversely (in the transverse direction perpendicular to themachine direction). Stretching the film leads to spatial orientation ofthe polymer chains. The longitudinal stretching can be carried out withthe aid of two rolls rotating at different speeds corresponding to thedesired stretching ratio. For the transverse stretching, an appropriatetenter frame can be used in which the film is clamped at the two edgesand then drawn towards the two sides at an elevated temperature.

Generally, stretching occurs at a temperature range of from about thesecond order transition temperature of the polymer to below thetemperature at which the polymer softens and melts. In one embodiment,for instance, longitudinal stretching can be carried out at atemperature in the range of from about 80° C. to about 130° C., whilethe transverse stretching can be carried out at a temperature in therange of from about 90° C. to about 150° C.

The longitudinal stretching ratio can generally be in the range of fromabout 2:1 to about 6:1, such as from about 2:1 to about 5:1. Thetransverse stretching ratio is also generally from about 2:1 to about6:1, such as from about 3:1 to about 5:1.

Where necessary, the film can be further heat treated after stretchingto lock-in the properties by further crystallizing the film. Thecrystallization imparts stability and good tensile properties to thefilm. Heat treatment, for instance, can generally be conducted at atemperature of from about 150° C. to about 250° C., such as from about190° C. to about 240° C. Coated films of the present disclosure, forinstance, can be exposed to heat at a temperature of from about 210° C.to about 250° C. for a period of from about 1 to about 20 seconds.

The polymer film can generally have a thickness of from about 2 mils toabout 15 mils, such as from about 2 mils to about 10 mils, such as fromabout 2 mils to about 8 mils.

In order to coat the film in accordance with the present disclosure, inone embodiment, the coating composition is applied to the film in-line.In particular, the coating composition is applied to the film while thefilm is being produced and before the film has been completely stretchedor heat set. For instance, in one embodiment, the coating compositioncan be applied to the polymer film after corona treatment and prior tostretch orientation. In one particular embodiment, for instance, thecoating composition can be applied to the film in-line by means of anaqueous dispersion after the longitudinal stretching step but prior tothe transverse stretching step.

In addition to in-line coating, the coating composition can also beapplied to the film off-line. Thus, the coating composition can beapplied to the film after the film has been produced and cooled. Whencoating both sides of the film, for instance, one side of the film canbe coated in-line, while the other side of the film can be coatedoff-line.

The resulting coated film can be used in numerous and differentapplications. As shown in FIGS. 1-3, for instance, in one embodiment thefilm can be laminated to a layer containing a cross-linkedethylene-vinyl acetate copolymer and used as a backing material for aphotovoltaic device. It should be appreciated, however, that the coatedfilm can be laminated to other various devices and structures.

EXAMPLE 1

18% barium sulfate-filled PET pellets were introduced into a singlescrew extruder where they were heated and compressed into a melt state.This melt was extruded through a slot die and cast onto a casting rollkept at about 20° C., where it solidified into an amorphous preliminaryfilm. The preliminary film was longitudinally stretched at 95° C. at astretching ratio of 3.6:1. The stretched film was passed under a coronatreater (Enercon Industries) and corona treated at 1.5 W/ft²·min.Subsequently, the longitudinally stretched film was coated by means of areverse gravure coating roll with an aqueous dispersion containing 20%solids content by weight, which contained 50% by weight Michem Prime4983R (Michelman) ethylene-acrylic acid copolymer dispersion and 50% byweight EPOCROS WS700 (Nippon Shokubai, Japan) oxazoline crosslinkerbased on the dried coating. The longitudinally dried film was then driedat around 100° C., and then stretched transversely in a stretching ratioof 4.3:1, so as to obtain a biaxially oriented film. The biaxiallystretched film was heat set at 230° C. The final film thickness was 250μm. The thickness of the dried coating was calculated from the solidscontent of the coating formulation, the coating thickness (wet) and thetransverse stretching factor to be 55 nm.

The coated film was incorporated in a laminate structures by theprocedures taught in this disclosure. Peel test laminates were producedusing both F806 EVA (Hangzhou First PV Material Company) and Photocap14520P/UF EVA (Specialized Technology Resources, Inc.). Peel tests wereconducted on test coupons as produced above. Test coupons were alsosubjected to HAST aging at 121° C. and 100% RH for 24 hr and then testedfor retained adhesive strength by the procedure taught in thisdisclosure. Average peel force for initial adhesion and retainedadhesion are summarized in Table 1. The coated film exhibits very goodadhesion to the cross-linked EVA layers initially and good retainedadhesion after the HAST aging procedure.

EXAMPLE 2

18% barium sulfate-filled PET pellets were introduced into a singlescrew extruder where they were heated and compressed into a melt state.This melt was extruded through a slot die and cast onto a casting rollkept at about 20° C., where it solidified into an amorphous preliminaryfilm. The preliminary film was longitudinally stretched at 95° C. at astretching ratio of 3.6:1. The stretched film was passed under a coronatreater (Enercon Industries) and corona treated at 1.5 W/ft²·min.Subsequently, the longitudinally stretched film was coated by means of areverse gravure coating roll with an aqueous dispersion containing 20%solids content by weight, which contained 50% by weight Michem Prime4983R (Michelman) ethylene-acrylic acid copolymer dispersion and 50% byweight EPOCROS WS700 (Nippon Shokubai, Japan) oxazoline crosslinkerbased on the dried coating. The longitudinally dried film was then driedat around 100° C., and then stretched transversely in a stretching ratioof 4.3:1, so as to obtain a biaxially oriented film. The biaxiallystretched film was heat set at 230° C. The final film thickness was 125μm. The thickness of the dried coating was calculated from the solidscontent of the coating formulation, the coating thickness (wet) and thetransverse stretching factor to be 55 nm.

The coated film was incorporated in a laminate structures by theprocedures taught in this disclosure. Peel test laminates were producedusing both F806 EVA (Hangzhou First PV Material Company) and Photocap14520P/UF EVA (Specialized Technology Resources, Inc.). Peel tests wereconducted on test coupons as produced above. Test coupons were alsosubjected to HAST aging at 121° C. and 100% RH for 24 hr and then testedfor retained adhesive strength by the procedure taught in thisdisclosure. Average peel force for initial adhesion and retainedadhesion are summarized in Table 1. The coated film exhibits very goodadhesion to the cross-linked EVA layers initially and good retainedadhesion after the HAST aging procedure.

EXAMPLE 3

18% barium sulfate-filled PET pellets were introduced into a singlescrew extruder where they were heated and compressed into a melt state.This melt was extruded through a slot die and cast onto a casting rollkept at about 20° C., where it solidified into an amorphous preliminaryfilm. The preliminary film was longitudinally stretched at 95° C. at astretching ratio of 3.6:1. The stretched film was passed under a coronatreater (Enercon Industries) and corona treated at 1.5 W/ft²·min.Subsequently, the longitudinally stretched film was coated by means of areverse gravure coating roll with an aqueous dispersion containing 22%solids content by weight, which contained 15% by weight PKHW-38 phenoxyresin (InChem Corp.) copolymer dispersion and 85% by weight EPOCROSWS700 (Nippon Shokubai, Japan) oxazoline crosslinker based on the driedcoating. The longitudinally dried film was then dried at around 100° C.,and then stretched transversely in a stretching ratio of 4.3:1, so as toobtain a biaxially oriented film. The biaxially stretched film was heatset at 230° C. The final film thickness was 250 μm, The thickness of thedried coating was calculated from the solids content of the coatingformulation, the coating thickness (wet) and the transverse stretchingfactor to be 55 nm.

The coated film was incorporated in a laminate structures by theprocedures taught in this disclosure. Peel test laminates were producedusing both F806 EVA (Hangzhou First PV Material Company) and Photocap14520P/UF EVA (Specialized Technology Resources, Inc.). Peel tests wereconducted on test coupons as produced above. Test coupons were alsosubjected to HAST aging at 121° C. and 100% RH for 24 hr and then testedfor retained adhesive strength by the procedure taught in thisdisclosure. Average peel force for initial adhesion and retainedadhesion are summarized in Table 1. The coated film exhibits very goodadhesion to the cross-linked EVA layers initially and good retainedadhesion after the HAST aging procedure.

EXAMPLE 4

18% barium sulfate-filled PET pellets were introduced into a singlescrew extruder where they were heated and compressed into a melt state.This melt was extruded through a slot die and cast onto a casting rollkept at about 20° C., where it solidified into an amorphous preliminaryfilm. The preliminary film was longitudinally stretched at 95° C. at astretching ratio of 3.6:1. The stretched film was passed under a coronatreater (Enercon Industries) and corona treated at 1.5 W/ft²·min.Subsequently, the longitudinally stretched film was coated by means of areverse gravure coating roll with an aqueous dispersion containing 22%solids content by weight, which contained 15% by weight PKHW-38 phenoxyresin (InChem Corp.) copolymer dispersion and 85% by weight EPOCROSWS700 (Nippon Shokubai, Japan) oxazoline crosslinker based on the driedcoating. The longitudinally dried film was then dried at around 100° C.,and then stretched transversely in a stretching ratio of 4.3:1, so as toobtain a biaxially oriented film. The biaxially stretched film was heatset at 230° C. The final film thickness was 125 μm. The thickness of thedried coating was calculated from the solids content of the coatingformulation, the coating thickness (wet) and the transverse stretchingfactor to be 55 nm.

The coated film was incorporated in a laminate structures by theprocedures taught in this disclosure. Peel test laminates were producedusing both F806 EVA (Hangzhou First PV Material Company) and Photocap14520P/UF EVA (Specialized Technology Resources, Inc.). Peel tests wereconducted on test coupons as produced above. Test coupons were alsosubjected to HAST aging at 121° C. and 100% RH for 24 hr and then testedfor retained adhesive strength by the procedure taught in thisdisclosure. Average peel force for initial adhesion and retainedadhesion are summarized in Table 1. The coated film exhibits very goodadhesion to the cross-linked EVA layers initially and good retainedadhesion after the HAST aging procedure.

COMPARATIVE EXAMPLE 1

WSAC PET film (Mitsubishi Polyester Films Inc.) a 250 μm, white coatedPET that is commercially sold as a backsheet material for PV modules wasincorporated into a laminate structure by the procedures taught in thisdisclosure. Peel test laminates were produced using both F806 EVA(Hangzhou First PV Material Company) and Photocap 14520P/UF EVA(Specialized Technology Resources, Inc.). Peel tests were conducted ontest coupons as produced above. Test coupons were also subjected to HASTaging at 121° C. and 100% RH for 24 hr and then tested for retainedadhesive strength by the procedure taught in this disclosure. Averagepeel force for initial adhesion and retained adhesion are summarized inTable 1. The coated film exhibits good adhesion to the cross-linked EVAlayers initially and but poor retained adhesion after the HAST agingprocedure.

HAST Aged 24 hr Intial Adhesion @121° C. Photocap F806 Photocap F806Film 14520P/UF FIRST 14520P/UF FIRST Thickness EVA EVA EVA EVA (microns)(N/cm) (N/cm) (N/cm) (N/cm) Example #1 1000 80 72 22 18 Example #2 50056 65 27 24 Example #3 1000 38 75 52 20 Example #4 500 37 50 44 25Comparative 1000 61 50 3 5 Example #1

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

What is claimed:
 1. A coated film comprising: a polyester film having afirst side and a second side, the polyester film having been biaxiallystretched; and a coating on the first side of the polyester film, thecoating comprising a polymer component and a cross-linking agent, thepolymer component comprising an alkylene carboxylic acid copolymer or aphenoxy resin, the cross-linking agent comprising an oxazoline, acarbodiimide, an epoxy, an isocyanate, or a melamine.
 2. A coated filmas defined claim 1, wherein the polymer component contained in thecoating comprises a ethylene acrylic acid copolymer or an ethylenemethacrylic acid copolymer.
 3. A coated film as defined in claim 2,wherein the cross-linking agent contained in the coating comprises anoxazoline polymer.
 4. A coated film as defined in claim 1, wherein thepolymer component contained in the coating comprises a phenoxy resin. 5.A coated film as defined in claim 1, wherein the first polymer filmcomprising the polyester contains pigment particles sufficient toprovide the first film with a white color.
 6. A coated film as definedin claim 5, wherein the white pigment particles comprise barium sulfateparticles and wherein the first polymer film has a Berger whiteness ofgreater than about
 70. 7. A coated film as defined in claim 1, whereinthe polyester film has a thickness of from about 2 mils to about 15 milsand the coating has a thickness of from about 10 nm to about 60 nm.
 8. Awhite polyester film comprising polyester and a coating on at least onesurface of said film, said coating comprising a cross-linking agent anda polymer component, the polymer component comprising an alkylenecarboxylic acid copolymer or a phenoxy resin, the cross-linking agentcomprising an oxazoline, a carbodiimide, an epoxy, an isocyanate, or amelamine.
 9. A white polyester film as defined in claim 8, wherein thepolymer component contained in the bonding layer comprises an ethyleneacrylic acid copolymer, or an ethylene methacrylic acid copolymer.
 10. Awhite polyester film as defined in claim 8, wherein the polymercomponent contained in the bonding layer comprises an ethylene acrylicacid copolymer.
 11. A white polyester film as defined in claim 10,wherein the cross-linking agent contained in the bonding layer comprisesan oxazoline polymer.
 12. A white polyester film as defined in claim 9,wherein the cross-linking agent comprises an oxazoline polymer formedfrom a. at least one oxazoline derivative according to one of thestructural formulae (I) to (III) and b. at least one further comonomer,

where the R₁, R₂, R₃ and R₄ radicals in the structural formulae (I) to(III) each independently represent hydrogen atoms, halogen atoms, alkylgroups, aralkyl groups, phenyl groups or substituted phenyl groups, andR₅ is a noncyclic radical with a polymerizable double bond.
 13. A whitepolyester film as defined in claim 12, wherein the comonomer (b)comprises one or more of the following compounds: methacrylic esters,unsaturated carboxylic acids, unsaturated nitrites, unsaturated amides,vinyl esters, vinyl ethers, alpha-olefins, halogenatedalpha,beta-unsaturated compounds or alpha,beta-unsaturated aromaticcompounds.
 14. A white polyester film as defined in claim 8, wherein thepolymer component contained in the bonding layer comprises a phenoxyresin.
 15. A white polyester film as defined in claim 8, wherein thepolyester film contains white pigment particles comprising bariumsulfate particles and wherein the polyester film has a Berger whitenessof greater than about
 70. 16. A white polyester film as defined in claim15, wherein the polymer component contained in the bonding layercomprises an ethylene acrylic acid copolymer and wherein thecross-linking agent comprises an oxazoline polymer.
 17. A photovoltaicdevice comprising: a solar cell having a back panel, the back panelcomprising the white polyester film of claim 8 laminated to across-linked ethylene vinyl acetate layer.
 18. A white polyester film asdefined in claim 8, wherein the first polymer film comprises a biaxiallystretched film made from polyethylene terephthalate.
 19. A whitepolyester film as defined in claim 8, wherein the polymer componentcomprises an ethylene acrylic acid copolymer and the cross-linking agentcomprises an oxazoline polymer, the polyester film having been biaxiallystretched, the coating having a thickness of from about 10 nm to about60 nm.
 20. A laminate comprising: a polyester film; a coating on thefirst side of the polyester film, the coating comprising a polymercomponent and a cross-linking agent, the polymer component comprising analkylene carboxylic acid copolymer or a phenoxy resin, the cross-linkingagent comprising an oxazoline, a carbodiimide, an epoxy, an isocyanate,or a melamine; and a polymer layer bonded to the polyester film, thecoating being positioned between the polyester film and polymer layer,the coating bonding the polymer layer to the polyester film.
 21. Alaminate as defined in claim 20, wherein the polymer layer comprises apolyolefin.
 22. A laminate as defined in claim 20, wherein the polymerlayer comprises a polyolefin film.
 23. A laminate as defined in claim21, wherein the polyolefin comprises a polyethylene or a polypropylene.24. A laminate as defined in claim 20, wherein the polymer componentcontained in the coating comprises a ethylene acrylic acid copolymer oran ethylene methacrylic acid copolymer, wherein the cross-linking agentcontained in the coating comprises an oxazoline polymer, and wherein thepolymer component contained in the coating comprises a phenoxy resin.